Microsensors will be developed for the in vivo detection of acetylcholine released into the extracellular fluid of the brain during stimulation of cholinergic neurons. The microsensors will be prepared by chemically modifying 3.5-mu m radius carbon fiber electrodes with a redox-polymer gel containing acetylcholinesterase choline oxidase and horseradish peroxidase. Electron transfer between peroxidase an e redox-polymer allows sensitive and selective electrochemical detection of the hydrogen peroxide formed in the gel as a product of the enzymatic oxidation of choline and acetylcholine. The microsensors will be used in vivo to monitor extracellular levels of acetylcholine in the cortex and striatum of rat brain during electrical and ionic (K+) stimulation of cholinergic neurons. Because the microsensors are small, they are well suited to monitoring evoked release. The minimal damage they cause to brain tissue during implantation allows them to be positioned very close to viable neuronal terminals, enabling the detection of released neurotransmitter before metabolism occurs in the extracellular fluid. The new microsensors will be used to elucidate the impact of dopaminergic drugs on evoked acetylcholine release from striatal cholinergic interneurons in intact animals. The link between dopaminergic and cholinergic elements in the striatum has been studied extensively in vitro but new in vivo techniques, such as the one proposed here, are needed to address persistent questions that remain unanswered. These in vivo studies will yield new insights into the neurobiology of central cholinergic systems which are implicated in a number of neurodegenerative disorders, including Parkinson's, Huntington's and Alzheimer's diseases.